Terahertz (THz) communications with a frequency band $0.1-10$ THz are envisioned as a promising solution to future high-speed wireless communication. Although with tens of gigahertz available bandwidth, THz signals suffer from severe free-spreading loss and molecular-absorption loss, which limit the wireless transmission distance. To compensate for the propagation loss, the ultra-massive multiple-input-multiple-output (UM-MIMO) can be applied to generate a high-gain directional beam by beamforming technologies. In this paper, a review of beamforming technologies for THz UM-MIMO systems is provided. Specifically, we first present the system model of THz UM-MIMO and identify its channel parameters and architecture types. Then, we illustrate the basic principles of beamforming via UM-MIMO and discuss the far-field and near-field assumptions in THz UM-MIMO. Moreover, an important beamforming strategy in THz band, i.e., beam training, is introduced wherein the beam training protocol and codebook design approaches are summarized. The intelligent-reflecting-surface (IRS)-assisted joint beamforming and multi-user beamforming in THz UM-MIMO systems are studied, respectively. The spatial-wideband effect and frequency-wideband effect in the THz beamforming are analyzed and the corresponding solutions are provided. Further, we present the corresponding fabrication techniques and illuminate the emerging applications benefiting from THz beamforming. Open challenges and future research directions on THz UM-MIMO systems are finally highlighted.

翻译：太赫兹（THz）通信（频段$0.1-10$ THz）被视为未来高速无线通信的一种有前景的解决方案。尽管拥有数十吉赫兹的可用带宽，太赫兹信号仍面临严重的自由传播损耗和分子吸收损耗，这限制了无线传输距离。为补偿传播损耗，可应用超大规模多输入多输出（UM-MIMO）技术，通过波束赋形生成高增益定向波束。本文对太赫兹UM-MIMO系统的波束赋形技术进行了综述。具体而言，我们首先给出了太赫兹UM-MIMO的系统模型，并确定了其信道参数和架构类型；随后阐述了通过UM-MIMO实现波束赋形的基本原理，并讨论了太赫兹UM-MIMO中的远场和近场假设。此外，介绍了一种太赫兹频段的重要波束赋形策略——波束训练，其中总结了波束训练协议和码本设计方法。分别研究了太赫兹UM-MIMO系统中智能反射面（IRS）辅助的联合波束赋形和多用户波束赋形。分析了太赫兹波束赋形中的空间宽带效应和频率宽带效应，并给出了相应解决方案。进一步，我们介绍了相关的制造工艺，并阐述了受益于太赫兹波束赋形的新兴应用。最后，强调了太赫兹UM-MIMO系统面临的开放挑战和未来研究方向。